Fixing roll for electrophotography

ABSTRACT

A fixing roll for electrophotography having a surface layer of a cured room-temperature-vulcanizing silicone rubber composition comprising a mixture consisting of two diorganopolysiloxanes, one of which is terminated at the chain ends with silanol groups and has a relatively high viscosity, and the other of which is terminated at the chain ends with trialkyl silyl groups and has a relatively low viscosity, an alkoxysilane or a partial hydrolysis-condensate thereof as the crosslinking agent, a metal salt of an organic acid as the reaction catalyst, and three kinds of inorganic fillers, namely, calcium carbonate, iron oxide, and titanium dioxide, and containing substantially no siliceous fillers. The fixing rolls have excellent properties including, in particular, the ability to provide a long copy-life to electrophotographic copying machines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improvements to an electrohotographic copyingmachine. Particularly, the invention relates to an improved hot-roll forfixing the toner images. Specifically, the invention relates toimprovements to the surface material of the fixing roll which is usefulin electrophotograpy.

2. Description of the Prior Art

There have been proposed several methods for flying reproduced images topaper in electrophotography. The most widely practiced method among themis the so-called "hot-roll method" using a heated roll to settle theimages formed by a toner containing a fusible resin as the binder on acopying paper by heat-fusing. The hot-roll method has some advantages,for example, higher efficiency of fixing, use of relatively lowtemperatures with decreased possibility of problems arising avoidance ofthe need to use special chemicals, and compact construction of thecopying machines. Hitherto, a metal roll whose surface is coated with afluoropolymer or a room-temperature-vulcanizing silicone rubber has beenused as the fixing roll for the hot-roll method because of theirexcellent thermal stability and releasability. See U.S. Pat. Nos.3,268,351 (Moser) and 3,498,596 (Van Dorn).

However, the fixing rolls coated with a fluoropolymer aredisadvantageous because it is necessary to repeatedly apply a releaseagent over the surface of the fluoropolymer layer during copying runs toaugment or make up the insufficient release property inherent in thefluoropolymer itself. Accordingly, the copying machine provided with thefluoropolymer-coated hot-roll should have an additional device for theapplication of the release agent, which results in rendering theconstruction of the machine more complicated.

On the other hand, the hot-rolls coated with aroom-temperature-vulcanizing silicone rubber can exhibit an excellentrelease property in the early part of the copying runs. Such releaseproperty then rapidly decreases and, as a result, the range oftemperature for the fixing step becomes steadily narrower, thus thefixing rolls coated with a room-temperature-vulcanizing silicone rubberhave proven to have a shorter copy-life and have to be subjected tofrequent renewal or replacement in order to avoid the so-called off-setphenomenon, i.e., transfer of the toner to the surface of the fixingroll, or winding of the copying paper around the roll.

Under the circumstances, improvements in the surface material of thefixing roll have been considered to be an important factor to increasethe efficiency of electrophotographic copying.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a fixing roll forelectrophotography which is serviceable without external feeding of anyrelease agent. Another object of the invention is to provide a fixingroll for electrophotography with a sufficient workability throughout awide temperature range. Still another object of the invention is toprovide a method for obtaining a fixing roll for electrophotographysuitable for a long copying run without loosing the release property ofthe non-sticky surface. A further object of the invention is to providea fixing roll useful in the hot-roll method with an unprecidented longcopy-life.

In accordance with the present invention, a fixing roll is provided withat least its outermost surface layer composed of a cured silicone rubbercomposition comprising two kinds of diorganopolysiloxanes, one of whichhaving diorganohydroxsilyl groups at the molecular terminals and acomparatively high viscosity which forms crosslinked structure bycuring, and the other having trialkylsilyl groups at the molecularterminals and a comparatively low viscosity which is effective to givereleasing property to the fixing roll, an alkoxy-containing silane orpartial hydrolysis-condensation product thereof serving as thecrosslinking agent, a metal salt of an organic acid serving as thecrosslinking catalyst, and three kinds of fillers i.e., powdery calciumcarbonate, iron oxide and titanium dioxide. In the silicone rubbercomposition, substantially no siliceous fillers are included, since thesiliceous fillers are harmful in the maintenance of the releasingproperty of the cured silicone rubber composition. The fixing rollhaving an outermost surface layer formed from such a silicone rubbercomposition can exhibit an outstandingly long copy-life as compared tothe prior art rolls, without replacement from time to time orapplication of any external releasing agent throughout a long period ofcopying operations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is based, in part, on the discovery that theroom-temperature-vulcanizing silicone rubber compositions forming atleast the outermost surface layer on the fixing roll a useful in fusingand fixing toner images to copy paper, overcome the problems involved inthe prior art fixing rolls, and contribute to a remarkably andunprecedented long copy-life, when certain non-siliceous fillers areadded, i.e., calcium carbonate, iron oxide and titanium dioxide. Suchcompositions also possess excellent releasing properties ornon-stickiness throughout a wise temperature range.

The compositions in accordance with the invention comprise

A. 100 parts by weight of a mixture consisting of

a. from 80 to 40% by weight of a diorganopolysiloxane terminated at bothchain ends with diorganohydroxysilyl groups directly bonded to theterminal silicon atoms, having a viscosity of from 100 to 500,000centistokes at 25° C and

b. from 20 to 60% by weight of a diorganopolysiloxane terminated at bothchain ends with trialkylsilyl groups, having a viscosity of from 5 to200 centistokes at 25° C,

B. from 0.5 to 10 parts by weight of an alkoxysilane represented by thegeneral formula

    R.sub.a.sup.1 Si(OR.sup.2).sub.4-a

where R¹ and R² are saturated or unsaturated monovalent hydrocarbongroups and a is 0 or 1, or a partial hydrolysis-condensate of thealkoxysilane,

C. from 0.1 to 5 parts by weight of a metal salt of an organic acid,

D. from 10 to 80 parts by weight of powdery calcium carbonate with anaverage particle diameter of not exceeding 10 μm,

E. from 10 to 80 parts by weight of powdery iron oxide with an averageparticle diameter of not exceeding 10 μm, and

F. from 10 to 80 parts by weight of powdery titanium dioxide with anaverage particle diameter of not exceeding 10 μm,

containing substantially no siliceous fillers.

Diorganopolysiloxane (a) in component (A) above are terminated at bothchain ends with diorganohydroxysilyl groups and has a viscosity of from100 to 500,000 centistokes, preferably, from 1,000 to 500,000centistokes, at 25° C. This diorganopolysiloxane works to cure thesilicone rubber composition, forming crosslinkages by dealcoholationcondensation of its silanol groups with the alkoxy groups of component(B). The organic groups bonded to the silicon atoms indiorganopolysiloxane (a) are exemplified by methyl, phenyl and vinylgroups and halogen substituted groups thereof. Preferably, all or almostall of the organic groups are methyl groups. Further,dimethylhydroxysilyl groups are preferred as the terminal groups of themolecular chain. The viscosity of diorganopolysiloxane (a) at 25° Cshould be in the range given above, since diorganopolysiloxanes having aviscosity outside the range do not give a sufficient workability andsatisfatory physical properties after cure to the silicone rubbercompositions.

Another diorganopolysiloxane (b) in component (A) is terminated at bothchain ends with trailkylsilyl groups and has a viscosity in the range offrom 5 to 200 centistokes at 25° C. This diorganopolysiloxane is anessential element for providing the silicone rubber compositions with areleasing property and nonstickiness. The organic groups bonded to thesilicon atoms in diorganopolysiloxane (b) are exemplified by methyl,phenyl and vinyl groups. Preferably, all or almost all of the organicgroups are methyl groups. Further, trimethylsilyl groups are preferredas the terminal groups of the molecular chain. Viscosities lower than 5centistokes at 25° C bring about a higher vapor pressure and hence moreevaporation loss to the diorganopolysiloxane during the fabrication offixing rolls and also the copying runs. On the other hand, viscositieshigher than 200 centistokes at 25° C result in giving a shortercopy-life to the fixing rolls concerned.

With respect to the amounts of diorganopolysiloxanes (a) and (b) to formcomponent (A) in accordance with the present invention, i.e., 80 to 40%by weight and 20 to 60% by weight, respectively, it is a generaltendency that diorganopolysiloxane (b) in an amount less than 20% byweight in component (A) will fail to give sufficiently strong anddurable releasing properties to the fixing rolls while, on the otherhand, diorganopolysiloxane (b) in an amount larger than 60% by weightwill adversely effect the curability of the resulting composition andthe physical properties of the composition after cure, and sometimesresult in a bleedthrough phenomenon.

Component (B) useful in the composition of the present invention is analkoxysilane represented by the following general formula

    R.sub.a.sup.1 Si(OR.sup.2).sub.4-a

where R¹ and R² are saturated or unsaturated monovalent hydrocarbongroups and a is 0 or 1, or a partial hydrolysis-condensate of thealkoxysilane. This component (B) is essentially used to formcrosslinkages by its reaction with the silanol groups contained in theabove-mentioned diorganopolysiloxane (a) and to cure the silicone rubbercomposition. A lower alkyl group such as a methyl or ethyl group ispreferred as group R¹ or R². Typical examples of the alkoxysilanes aretetraethoxysilane, methyl trimethoxysilane, methyl triethoxysilane andthe like. The partial hydrolysis-condensates from these alkoxysilanesmay be prepared by a known method using controlled amounts of water andan acid catalyst, provided that the degree of the partial hydrolysiscondensation should naturally be such that the compatibility of theproduct with component (A) will not be impaired and at least 3 alkoxygroups remain in a molecule of the same product. For such a partialhydrolysis-condensate, the commercially available polyethyl silicate maybe employed.

Component (B) should be used in an amount from 0.5 to 10 parts by weightper 100 parts by weight of component (A) in the composition. When theamount is less than 0.5 parts by weight, a sufficient cure of thecomposition cannot be attained, while the amounts larger than 10 partsby weight tend to form excessive crosslinkages, resulting in increasingthe hardness of the surface layer prepared from the resultingcomposition making it unfit for the desired fixing rolls.

The metal salt of an organic acid as component (C) useful in thecomposition acts as a catalyst to accelerate dealcoholation condensationbetween the above-mentioned diorganopolysiloxane (a) and component (B),and may be selected from among any conventional catalysts suitable forthe purpose. Most preferred are, for example, dibutyltin dilaurate,dibutyltin dioctoate and the like because of their compatibility withcomponents (A) and (B) and the ease of adjusting the rate of thecondensation reaction within a suitable range. The amount of component(C) in the silicone rubber composition may be determined in a wide rangein accordance with the desired velocity of curing of the compositionbut, in general, it is from 0.1 to 5 parts by weight per 100 parts byweight of component (A).

Powdery calcium carbonate as component (D), powdery iron oxide ascomponent (E) and powdery titanium dioxide as component (F) are all theinorganic fillers that are useful in the silicone rubber compositionaccording to the present invention. Conventionally, siliceous fillers,such as, diatomaceous earth, fume silica, precipitated silica, finepowder of quartz and the like have been considered to be the mostsuitable inorganic fillers for the formulation of silicone rubbercompositions and indispensable components to yield a cured siliconerubber having high mechanical strengths because of their markedefficiency for reinforcement. Nonetheless, it has been discovered by theextensive studies of the inventors that those siliceous fillers areextremely harmful when used in the silicone rubber composition for thefixing roll, while the three inorganic fillers, i.e., theabove-mentioned components (D), (E) and (F), when used in specificamounts give satisfactory physical properties to the silicone rubbercomposition and a very long copy-life to the fixing roll.

As described in the above, the inclusion of siliceous fillers into thesilicone rubber composition to be applied to the fixing roll, eventhough in small amounts, results in a remarked reduction of thecopy-life of the fixing roll. The reason for this is not altogetherclear, but may presumably be understood by the following mechanism.

The resinous component contained in the toner readily transfers to thesurface of the silicone rubber layer owing to the high polarity of thesilanol groups which are generally present in large numbers at thesurfaces of the siliceous filler particles. The resinous component may,on the other hand, form chemical bonds with the surfaces of thesiliceous filler particles in the silicone rubber composition, dependingon the kind of the resin. Therefore, the toner tends to accumulate onthe surface of the fixing roll, resulting in a quick reduction of thereleasing property. The number of the silanol groups may be reduced byblocking them with trimethylsilyl groups in a treatment using, forexample, trimethylchlorosilane. As such, complete removal of the silanolgroups As such, complete removal of the silanol groups cannot beattained, and it only to slightly migitates the above-described harmfuleffects. Furthermore, it is presumed that, due to the fact thatsiliceous fillers have a high affinity with organopolysiloxanes, thesiliceous fillers work to hinder the migration of diorganopolysiloxane(b) toward the surface of the cured silicone rubber composition layer,resulting in rendering it difficult for diorganopolysiloxane (b) toexhibit its releasing properties.

on the contrary, the desirable use of diorganopolysiloxane (b) incomponent (A) is suitably preserved by simultaneously using inorganicfiller components (D), (E) and (F), and the resulting cured siliconerubber compositions have sufficiently good mechanical strengths for theintended purpose. The average particle size of these inorganic fillersmust not exceed 10 μm, preferably a few micrometers or smaller, sincecoarser particles bring about adverse effects on the mechanicalproperties of the cured organopolysiloxane compositions. Further, thesuitable amounts of the fillers in the desired composition are all from10 to 80 parts by weight per 100 parts by weight of component (A). Nofixing roll with satisfactory performance can not be obtained when thosefiller components are used in amounts outside the above range.

Besides components (A) to (F), the silicone rubber compositions for thefixing roll according to the present invention may contain various otherconventional additives, such as, thermal-resistance improvers,antioxidants, coloring agents, and surface treatment agents for theinorganic fillers. Small amounts of other inorganic fillers exceptsiliceous fillers may also be added.

The room-temperature-vulcanizing silicone rubber compositions useful forthe fabrication of the fixing roll in accordance with the presentinvention are easily prepared by merely mixing components (A) to (F) byany adequate mixer at room temperature or at a slightly elevatedtemperature. The order of adding these components is not critical,provided however if a long shelf-life of the composition is desired, itis recommended to effect the addition of (C) just before the finishedcomposition is applied to the fixing roll.

The thickness of the layer of the room-temperature-vulcanizing siliconerubber composition provided on the fixing roll surface is at least 1 mm,or preferably a few millimeters. The curing of the composition takesplace at room temperature, but may be accelerated by heat for purposesof obtaining a higher efficiency of working.

The preparation of the desired fixing rolls using the silicone rubbercompositions in accordance with the present invention is notparticularly limited; any of the conventional methods can besatisfactorily applied.

The following examples illustrate the present invention. All "parts" inthe examples are parts by weight. "Copy-life" indicating the number ofsheets of paper treated by one particular fixing roll in the examples isdetermined by the following manner.

With an electrophotographic copying machine, Type PPC-900, made by RicohCompany, Ltd., Japan, a fixing roll prepared in accordance with each ofthe examples was mounted in place of the existing standard fixing roll,and a copy run was carried out under the test conditions given belowuntil the copying paper started to wind round the fixing roll, then thenumber of sheets of the copy paper already fed was counted to denote thecopy-life of the fixing roll.

    ______________________________________    Test Conditions:    Temperature for fixing                   165° C    Contact pressure for    fixing         7 kg    Original pattern to                   A sheet with about 20% of    be copied      images areas.    Copying density                   About 1.0, measured by                   Macbeth Densitometer, product                   of Macbeth Company    Copying paper  Sheets of paper specified for                   a PPC-900-type copying machine,                   product of Ricoh Company, sized                   B4 according to Japanese                   Industrial Standard, i.e.,                   257 × 364 mm    Copying speed  15 sheets per minute    Toner          A toner specified for the PPC-                   900-type copying machine    ______________________________________

EXAMPLE 1

Room-temperature-vulcanizing silicone rubber compositions were preparedby mixing and kneading 70 parts of dimethylpolysiloxane terminated atboth chain ends with dimethylhydroxysilyl groups, having a viscosity of5,000 centistokes at 25° C, 30 parts of dimethylpolysiloxane terminatedat both chain ends with trimethylsilyl groups, having a viscosity of 20centistokes at 25° C, 3 parts of polyethyl silicate, 0.5 part ofdibutyltin dilaurate, and the varying amounts of powdery calciumcarbonate with an average particle diameter of 1.2 μm, powdery ironoxide with an averge particle diameter of 0.8 μm and powdery titaniumdioxide with an average particle diameter of 0.3 μm, as indicated inTable I. With each of the compositions thus prepared (Samples Nos. 1-5)the surface of a roll made of stainless steel was coated in a thicknessof about 3 mm, and then the cured coating was finished by grinding. Thethus obtained rolls were used as the fixing rolls for the purpose ofthis example, and each fixing roll was tested for its copy-life with theresults as set out in Table I.

                  Table I    ______________________________________               Sample No.               1     2       3       4     5    ______________________________________    Calcium carbonate,                 35      20      30    40    20    parts    Iron oxide,  30      20      30    15    60    parts    Titanium dioxide,                 25      20      30    10    40    parts    Copy-life, sheets                 30,000  27,500  30,500                                       25,000                                             34,000    ______________________________________

EXAMPLE 2

Room-temperature-vulcanizing silicone rubber compositions (Samples Nos.6 and 7) were prepared in the same manner as in the formation of SampleNo. 1 in Example 1 except the first and second dimethylpolysiloxaneswere used in amounts of 80 and 20 parts (for Sample 6) or 50 and 50parts (for Sample 7),, instead of 70 and 30 parts. The fixing rollobtained in the same manner as in Example 1 with each of thesecompositions exhibited the following copy-life.

    ______________________________________    With Sample No. 6 24,000 sheets    With Sample No. 7 35,000 sheets    ______________________________________

EXAMPLE 3

Fixing rolls were prepared according to the same manner as in Example 1using the same room-temperature-vulcanizing silicone rubber compositionas of Sample No. 1 in Example 1 except the second dimethylpolysiloxane(terminated at both chain ends with trimethylsilyl groups) had aviscosity at 25° C of 10 centistokes (for Sample No. 8), 50 centistokes(for Sample No. 9) or 100 centistokes (for Sample No. 10), instead of 20centistokes. These fixing rolls exhibited the following copy-life.

    ______________________________________    With Sample No. 8 26,000 sheets    With Sample No. 9 27,500 sheets     With Sample No. 10                      25,000 sheets    ______________________________________

Another fixing roll was prepared with the sameroom-temperature-vulcanizing silicone rubber composition exceptmethylphyenylpolysiloxane terminated with trimethylsilyl groups, havinga viscosity of 100 centistokes at 25° C and consisting of 95 mole % ofdimethylsiloxane units and 5 mole % of diphenylsiloxane units wereincluded in place of the dimethylpolysiloxane employed in Sample Nos.8-10. This fixing roll exhibited a copy-life of 24,000 sheets.

EXAMPLE 4

This is a comparative example. Fixing rolls were prepared according tothe same manner as in Example 1 using the sameroom-temperature-vulcanizing compositions as Sample No. 1 in Example 1except that any one or two of the inorganic fillers, i.e., calciumcarbonate, iron oxide and titanium dioxide, were omitted (Sample Nos.12-15) as set out in Table II together with their respective amountsused. Each fixing roll exhibited a copy-life as shown in Table II.

                  Table II    ______________________________________               Sample No.               12     13       14       15*    ______________________________________    Calcium carbonate,                 40       60       45     Nil    parts    Iron oxide,  Nil      Nil      45     60    Titanium dioxide,                 30       Nil      Nil    Nil    parts    Copy-life, sheets                 15,000   20,000   19,500 15,000    ______________________________________     *Without finishing by grinding of its cured surface.

EXAMPLE 5

This is another comparative example. Fixing rolls were preparedaccording to the same manner as in Example 1 usingroom-temperature-vulcanizing silicone rubber compositions (Sample Nos.16-25) consisting of the same diorganopolysiloxane, polyethyl silicateand dibutyltin dilaurate in the same amounts as in Sample No. 1 ofExample 1 and the varying inorganic filler or fillers as indicated inTable III together with their respective amounts used. Each fixing rollexhibited a copy-life as shown in the same table.

                                      Table III    __________________________________________________________________________                Sample No.                16  17  18  19  20  21  22  23  24  25    __________________________________________________________________________    Diatomaceous earth,                60  0   0   35  40  20  15  20  0   0    parts    Fume silica*,                0   25  0   0   0   0   0   5   0   0    parts    Powdery quartz**,                0   0   90  0   0   0   0   0   45  30    parts    Zinc oxide, 0   0   0   0   0   0   0   0   0   10    parts    Calcium carbonate,                0   0   0   25  0   25  20  20  20  15    parts    Iron oxide, 0   0   0   0   60  0   25  0   0   15    parts    Titanium dioxide,                0   0   0   0   0   25  15  20  15  15    parts    Copy-life, sheets                2,600                    2,400                        6,500                            3,500                                3,000                                    9,000                                        10,000                                            7,000                                                8,000                                                    9,800    __________________________________________________________________________     *Surface-treated with trimethylchlorosilane. Specific surface area 230     m.sup.2 /g.     **Average particle diameter 5 μm.

EXAMPLE 6

This is a further comparative example. Using the sameroom-temperature-vulcanizing silicone rubber composition as Sample 1 ofExample 1 except that the first and second dimethylpolysiloxanes wereincluded in the proportion of 90 and 10 parts, instead of 70 and 30parts, a fixing roll was prepared. The fixing roll was tested for itscopy-life, to cover only 8,000 sheets.

Further, on the other hand, a similar formulation of theroom-temperature-vulcanizing silicone rubber composition was made usingthe first and second dimethylpolysiloxanes in the proportion of 30 and70 parts, and the resulting formulation was too soft to be suited forthe preparation of the desired fixing roll.

EXAMPLE 7

This is a still further comparative example. A fixing roll was preparedaccording to the same manner as in Example 1, using the sameroom-temperature-vulcanizing silicone rubber composition as Sample 1 ofExample 1 except that the second dimethylpolysiloxne (terminated at bothchain ends with trimethylsilyl groups) had a viscosity of 1,000centistokes at 25° C, instead of 20 centistokes at 25° C: The fixingroll exhibited a copy-life covering 12,000 sheets.

Apart from the above, a similar room-temperature-vulcanizing siliconerubber composition was formulated with 100 parts of the firstdimethylpolysiloxane terminated at both chain ends withdimethylhydroxysilyl groups, having a viscosity of 5,000 centistokes at25° C, in the total absence of the second dimethylpolysiloxane. Withthis composition a fixing roll was prepared, and the fixing rollexhibited a copy-life to cover 3,000 sheets.

What is claimed is:
 1. A fixing roll for electrophotography having asurface layer with a thickness of at least 1 mm composed of a curedroom-temperature vulcanizing silicone rubber composition comprisingA.100 parts by weight of a mixture consisting ofa. from 80 to 40% byweight of a diorganopolysiloxane terminated at both chain ends withdiorganohydroxysilyl groups directly bonded to the terminal siliconatoms, having a viscosity of from 100 to 500,000 centistokes at 25° Cand b. from 20 to 60% by weight of a diorganopolysiloxane terminated atboth chain ends with trialkylsilyl groups, having a viscosity of from 5to 200 centistokes at 25° C, B. from 0.5 to 10 parts by weight of analkoxysilane represented by the general formula

    R.sub.a.sup.1 Si(OR.sup.2).sub.4-a

where R¹ and R² are saturated or unsaturated monovalent hydrocarbongroups and a is 0 or 1, or a partial hydrolysis-condensate of thealkoxysilane, C. from 0.1 to 5 parts by weight of a metal salt of anorganic acid, D. from 10 to 80 parts by weight of powdery calciumcarbonate with an average particle diameter of not exceeding 10 μm, E.from 10 to 80 parts by weight of powdery iron oxide with an averageparticlediameter of not exceeding 10 μm, and F. from 10 to 80 parts byweight of powdery titanium dioxide with an average particle diameter ofnot exceeding 10 μm.
 2. The fixing roll for electrophotography asclaimed in claim 1 wherein diorganopolysiloxane (a) has organic groupsselected from methyl, phenyl and vinyl groups and halogen substitutedgroups thereof bonded directly to silicon atoms.
 3. The fixing roll forelectrophotography as claimed in claim 1 wherein the terminal groups ofdiorganopolysiloxne (a) are dimethylhydroxysilyl groups.
 4. The fixingroll for electrophotography as claimed in claim 1 whereindiorganopolysiloxane (b) has organic groups selected from methyl, phenyland vinyl groups bonded directly to silicon atoms.
 5. The fixing rollfor electrophotography as claimed in claim 1 wherein the terminal groupsof diorganopolysiloxane (b) are trimethylsilyl groups.
 6. The fixingroll for electrophotography as claimed in claim 1 wherein said R¹ or R²group is a lower alkyl group selected from methyl and ethyl groups. 7.The fixing roll for electrophotography as claimed in claim 1 whereinsaid metal salt of an organic acid (C) is selected from dibutyltindilaurate and dibutyltin dioctoate.